The basis of modern MRI systems is a magnet whose field has superimposed upon it gradient magnetic fields typically in X Y Z co-ordinates for spatial resolution, in an imaging volume. The coils for generating these gradient fields are energised by pulses of electric current which may comprise a rapid rise, then a flat top and then a rapid fall, followed by a reversal, after which the cycle repeats. The amplitude of the current pulses depends upon the strength of the gradient field required in the imaging volume together with the associated efficiency of the coil design. In addition the rise and fall time of the current pulses will depend upon the inductance of the gradient coil and the voltage available to drive the current during the ramp. With existing coil designs for whole body imaging, currents in the range of 100-300 amps (and most typically in the region of 300 amps) are generally required at voltages up to 300V depending on the rise-time required. For very fast imaging these voltages and currents may be significantly higher. At present most MRI systems achieve this using linear power amplifiers, an example being those manufactured by the Techron Corporation.
Such an amplifier is essentially a modified form of a linear, high power audio amplifier and is able to deliver peak currents of 150 amps at a maximum voltage of +/-150V with a bandwidth of DC-50 KHz. These amplifiers can be paralleled to increase their current capability and wired in a series or bridge arrangement to increase the output voltage swing, although combining amplifiers in this way adds considerably to cost and complexity. Each amplifier must also be powered from a three-phase mains supply and when driven at full power can demand high peak currents from the mains, presenting a poor power factor to the public electricity supply. If these amplifiers are powered from a diesel generator for mobile use then a special design of alternator and voltage regulator is required in order to cope with the current pulses demanded. At present rates (1997) each amplifier costs .English Pound.4500 and so to achieve a gradient drive of 300 A/300V for one axis four amplifiers would be required at a cost of .English Pound.18000. For all three axes this would total .English Pound.54000.
Two different systems using high power amplifiers such as the Techron are described in EP-A-0460894 and EP-A-0460895. Both specifications describe circuits intended to speed up the rise and fall times of the current pulses needed to generate the gradient magnetic fields in MRI systems; in particular the rise-time of the current pulse is improved by switching in a separate high voltage supply to the coil during the ramp via a switching bridge. This has been designed as a retrofit to existing amplifier systems in order to improve their performance particularly for high-speed imaging, but nevertheless adds to cost and complexity.